Variable Restrictiveness Exhaust - Meet Mr Helmholz

[Robert Harris]

The majordomo deleted portion proposed active acoustical tuning using a
"microphone" or a sensitive pressure sensor well filtered and positioned near
the valve to observe the exhaust acoustical pressure and in conjunction with a
camshaft angle sensor, we would have the feedback mechanism to actively
measure and time the arrival of the negative going pulse.

While the Helmholz model may not be perfect, it gives us reasonable bounds to
develope within without getting into magic.  This model tells us that we can
change the frequency of the resonance by changing the volume and the volume
does not have to flow gas.   Thus the slippy pipe.

In reality, any reasonable way of varying the volume of a connected chamber
will allow us to move the resonance to the current rpm.  Multiple stubs
connected by butterfly valves may work as well - depending on the broadness
"Q" of the system.  

In the Helmholz model, the final tuning is done by the varying the length and
area of the tuning pipe attached to the volume.  This corresponds to the
exhaust system past the collector "resonating" volume.  Shortening the pipe by
a "cutout" properly placed could shorten the effective length of the
resonating pipe, raise the resonant frequency and thus extend the active
bandwidth again.  

Again, with a Helmholz model, we can approximately predict the effect of
changes either in the resonating volume or the tuning pipe.  With the
acoustical pressure vs camshaft angle measurement, we have a practical way of
determining the effectiveness.  

For those that followed the link, you see how effective varying the
temperature can be.  Varying the temperature by water injection into the
resonating volume works by varying the local speed of sound with the system.

Since the sole purpose of the water is to vary the resonance by varying the
temperature it becomes very interesting on how to administer the water.
Probably the simplest  is to vary the temperature of the gas in the collector.
But would timed injection into the exhaust port just before the exhaust valve
opens be more effective or just trailing the major pulse or perhaps even into
the exhaust pulse itself.  ????  Floors open.

Date: Mon, 17 May 1999 22:00:40 -0500
From: Tom Sharpe <twsharpe at mtco.com>
Subject: Re: Variable Restrictiveness Exhaust - Meet Mr Helmholz

Deleted Stuff

>Robert, I really like the slippy pipes... could you figure out the length from the
>sound produced? Sort of a knock sensor approach...

>or use lookup tables like fuel, spark, exhaust position??  Just trying to simplify
>the process..

>Do we need a stepper motor headlight actuator....  Tom S

------------------------------